This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-333375, filed Nov. 24, 1999; and No. 11-371763, filed Dec. 27, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a mobile radio terminal apparatus in the Code Division Multiple Access (CDMA) scheme for use in a mobile radio communication system such as a mobile telephone system or a portable telephone system.
Recently, a mobile radio communication system employing the CDMA scheme has been noticed. This system employs the spread spectrum communication scheme as the communication scheme thereof.
A conventional mobile radio terminal apparatus in the mobile communication system will be explained with reference to FIG. 1. The receiving system relating to the invention will be mainly explained here.
A transmitting device 12 modulates the transmission data of the speech communication, data communication and the like in the digital modulation scheme such as the PSK modulation and spreads the modulated data to a broad-band baseband signal by using a spread code.
The transmitting device 12 up-converts the baseband signal to a radio-frequency signal, inputs the signal to a first antenna 10 via a duplexer 11, and irradiates the signal into the space from the first antenna 10 to transmit the signal to a base station (not shown).
On the other hand, the radio signal transmitted from the base station is received by the first antenna 10 and input to a receiving device 13 via the duplexer 11. The receiving device 13 is composed of a radio circuit 14, an intermediate frequency circuit 15 and a RAKE receiver 16.
First, in the radio circuit 14, the radio signal received from the duplexer 11 is input to an attenuator 14a, which attenuates the signal at a preset amount.
The signal passing through the attenuator 14a is amplified up to a predetermined level by an amplifier 14b, mixed with a signal generated by a frequency synthesizer 14d in a mixer 14c and then down-converted to an intermediate frequency.
The signal down-converted to the intermediate frequency is input to the intermediate frequency circuit 15 and amplified up to a predetermined level by an amplifier 15a. A desired band, of the result of the amplification, passes through a band-pass filter (BPF) 15b and is input to a mixer 15c. 
In the mixer 15c, the signal which has passed through the band-pass filter 15b and a signal generated by a frequency synthesizer 15d are mixed and converted to a baseband signal. The baseband signal converted to a digital signal by an A/D converter (A/D) 15e and input to the RAKE receiver 16.
The RAKE receiver 16 is composed of a searcher 16a, fingers 16b, 16c and 16d, and a symbol combiner 16e. The digital signal is input to the searcher 16a and also to the fingers 16b, 16c and 16d. 
The searcher 16a despreads the digital signal at various timings by using the same spread code as that used for the spreading on the transmitting side, in order to execute the detection of signals arriving in a plurality of paths from the base station to its own terminal, or what is called multi-pass detection.
The searcher 16a obtains Ec/Io (Io: spectral density of overall electric power in the receiving range, Ec: pilot energy stored during a PN chip period) of a plurality of paths and a delayed time difference (delay profile) of these paths from the result of the despreading. On the basis of these, the searcher 16a obtains a receiving timing (despread timing) of a path suitable for the reception and assigns it to each of the fingers 16b, 16c and 16d. 
The fingers 16b, 16c and 16d despread the digital signal by using the same spread code as that used in the spreading on the transmitting side, at the despreading timings assigned by the searcher 16a. 
The symbol combiner 16e symbolically combines the multi-pass components respectively despread by fingers 16b, 16c and 16d by considering the despreading timings assigned to the respective fingers 16b, 16c and 16d. 
The signal symbolically combined in the symbol combiner 16e is subjected to the demodulation corresponding to the digital modulation of the transmitting side in a signal processing section 17 of the rear stage and thereby the receiving data is reproduced.
In this mobile radio terminal apparatus, a control section (not shown) executes the control operation for the handoff, in accordance with the value of Ec/Io of the pilot signal obtained in the searcher 16a. 
When the intermittent reception is executed in a reception standby state under this control, if the value of Ec/Io satisfies conditions that (1) the value is smaller than a predetermined value, (2) the value is more deteriorated than a predetermined value as compared with the previous receiving time, (3) a difference between the value and Ec/Io of a neighboring base station becomes within a predetermined value, and the like, Ec/Io of the pilot signal from the other base station is measured to execute the hand off. If Ec/Io of the pilot signal of the other base station becomes greater than the predetermined value, the signal is handed off to the other base station.
If Ec/Io of the pilot signal of the currently connected base station becomes smaller than the predetermined value and if Ec/Io of the pilot signal of the neighboring base station becomes greater than the predetermined value while the mobile radio terminal apparatus executes the communication, the mobile radio terminal apparatus executes the hand off while having communication with a plurality of base stations including the base station satisfying the determined conditions.
The above-described CDMA scheme have advantages that (1) the scheme is superior in the secrecy of communication as the spread spectrum technique is employed, (2) the scheme has high resistance to fading as the Rake receiving scheme is employed, (3) the scheme is capable of stable handoff including no instantaneous interruption of communication or what is called soft handoff, and the like.
In the conventional mobile radio terminal apparatus in the CDMA scheme, however, the multi-pass resolution of the searcher 16a is determined in accordance with the reciprocal of the chip rate, i.e. one PN chip period. Therefore, if the extension of the multi-pass delay time is smaller than the resolution, the multi-pass components cannot be separated respectively at the fingers 16b, 16c and 16d or symbolically synthesized.
That is, extension of the delay time of the multipass (direct wave, first reflected wave and second reflected wave) received by the fingers 16b, 16c and 16d is small, and the Rake reception cannot be executed if the multipass is not delayed by more than the reciprocal of the chip rate.
Thus the reflected wave which is more delayed than the direct wave cannot be reproduced in accordance with the delay time difference in the multipass. Therefore, there is a problem that the resistance to fading is deteriorated and stable communication quality cannot be maintained. This problem arises more remarkably in a low-speed motion such as walking or a stationary condition than in a high-speed motion.
In addition, if the multipass fading occurs under the condition that the Rake reception cannot be executed as the multipass delay time is small, so as to detect a predetermined number of frame errors in the pilot signal from the current base station even temporarily, it is estimated that the system is lost and the process starts from the system capture.
The reception is executed at a location where the receiving energy is accidentally deteriorated due to the influence of the fading as shown in FIG. 2.
As described above, if it is estimated that the system is lost and the process starts again from the system capture, the communication will be interrupted during the communication time. At the standby time, xe2x80x9cout of communication rangexe2x80x9d is displayed and it will take much time to become in the standby condition. These cause the battery to be wasted unnecessarily.
The first object of the present invention is to provide a mobile radio terminal apparatus in the CDMA scheme capable of maintaining the stable communication quality even in a radio wave atmosphere in which terrible fading occurs.
To achieve the first object, the mobile radio terminal apparatus of the present invention is capable of making radio connection in the CDMA scheme to one of a plurality of radio base stations connectable to a communication network to conduct communication with a communication station in the communication network, and comprises two antennas, antenna selection means for selecting one of the two antennas as an antenna which is to be used, in accordance with a control signal, receiving means for receiving a signal transmitted from the radio base station via the antenna selected by the antenna selection means, receiving energy estimating means for obtaining a ratio of a pilot energy stored in a preset period to a spectral density of entire electric power in a receiving range, of the signal received by the receiving means, and antenna change control means, in a case where receiving quality of the signal received by the receiving means is lower than a predetermined value after an incoming call signal is received from the radio base stations, at the time of transmitting an outgoing signal to the radio base stations, or during the communication with the radio base stations, for supplying the control signal to the antenna selection means, changing the antenna in use from the antenna selected by the antenna selection means to the other antenna for the reception, comparing the ratios obtained by the receiving energy estimating means before and after the change of the antennas, and supplying the control signal to select the antenna having the larger ratio to the antenna selection means.
In the above-constituted mobile radio terminal apparatus, if the receiving quality is lower than the predetermined value after an incoming signal is received from the radio base station, at the time of transmitting an outgoing signal to the radio base station, or during the communication with the radio base station, the control signal is supplied to the antenna selection means, the antenna in use is changed from the antenna selected by the antenna selection means to the other antenna for the reception, the ratios obtained by the receiving energy estimating means are compared before and after the change of the antennas, and the antenna having the larger ratio is selected.
Thus, according to the above-constituted mobile radio terminal apparatus, the antenna having the larger ratio is selected even in a radio wave atmosphere in which terrible fading occurs, at the outgoing or incoming time or at the communication time, and therefore the stable communication quality can be maintained.
Further, to achieve the first object, the mobile radio terminal apparatus of the present invention is capable of making radio connection in the CDMA scheme to one of a plurality of radio base stations connectable to a communication network to conduct communication with a communication station in the communication network, and comprises two antennas, antenna selection means for selecting one of the two antennas as an antenna which is to be used, in accordance with a control signal, receiving energy estimating means for obtaining a ratio of a pilot energy stored in a preset period to a spectral density of entire electric power in a receiving range, of the signal received via the antenna selected by the antenna selection means, and antenna change control means for sequentially changing the two antennas and executing the reception via the respective antennas, prior to a receiving timing preliminarily assigned to the own apparatus at an incoming call standby time, comparing the ratios obtained by the receiving energy estimating means before and after the change of the antennas, and supplying the control signal to select the antenna having the larger ratio to the antenna selection means.
In the above-constituted mobile radio terminal apparatus, two antennas are sequentially changed and the reception is executed via the respective antennas, prior to a receiving timing preliminarily assigned to the own apparatus at the incoming call standby time, the ratios obtained by the receiving energy estimating means before and after the change of the antennas are compared, and the antenna having the larger ratio is selected.
Thus, according to the above-constituted mobile radio terminal apparatus, the antenna having the larger ratio is selected even in a radio wave atmosphere in which terrible fading occurs, at the incoming call standby time, and therefore the stable communication quality can be maintained and the first object can be achieved.
The second object of the present invention is to provide a mobile radio terminal apparatus capable of maintaining the stable communication quality and preventing xe2x80x9csystem lostxe2x80x9d even in a radio wave atmosphere in which the multipass delay time is short and terrible fading occurs.
To achieve the second object, the mobile radio terminal apparatus of the present invention is capable of making radio connection in the CDMA scheme to one of a plurality of radio base stations connectable to a communication network to conduct communication with a communication station in the communication network, and comprises two antennas, antenna selection means for selecting one of the two antennas as an antenna which is to be used, in accordance with a control signal, frame error estimating means for demodulating a signal received via the antenna selected by the antenna selection means and estimating whether or not a predetermined number of frame errors are successively generated in the result of the demodulation, and antenna change control means, in a case where it is estimated by the frame error estimating means that a predetermined number of frame errors are successively generated, for supplying the control signal to the antenna selection means and changing the antenna in use from the antenna selected by the antenna selection means to the other antenna.
In addition, a mobile radio terminal apparatus according to the present invention is capable of making radio connection in the CDMA scheme to one of a plurality of radio base stations connectable to one of a communication network to conduct communication with a communication station in the communication network, and comprises two antennas, antenna selection means for selecting one of the two antennas as an antenna which is to be used, in accordance with a control signal, frame error estimating means for demodulating a signal received via the antenna selected by the antenna selection means and estimating whether or not frame errors are successively generated in the result of the demodulation in more than a predetermined time, and antenna change control means, in a case where it is estimated by the frame error estimating means that frame errors are successively generated during more than a predetermined period, for supplying the control signal to the antenna selection means and changing the antenna in use from the antenna selected by the antenna selection means to the other antenna.
According to the above-constituted mobile radio terminal apparatus, when more than a predetermined number of frame errors of the receiving signal are successively generated or frame errors are successively generated in more than a predetermined time, the antenna in use is changed from the antenna selected by the antenna selection means to the other antenna to execute the reception.
That is, when the frame errors of the receiving signal are successively generated in more than a predetermined time or more than a predetermined number of frame errors are successively generated, even temporarily, by the multipass fading, so that stable communication quality cannot be maintained, the reception can be executed by employing the other antenna.
Thus, as the position where the RF signal from the space is received is changed by the distance between two antennas, the RF signal receiving little influence of the fading can be received via the other antenna.
Therefore, according to the above-constituted mobile radio terminal apparatus, it is possible to maintain the stable communication quality and prevent xe2x80x9csystem lostxe2x80x9d even in a radio wave atmosphere in which the multipass delay time is short and terrible fading occurs, and thereby the second object of the present invention can be achieved.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.